The invention relates to nuclear medicine, and more particularly relates to nuclear medicine SPECT studies. In its most immediate sense, the invention relates to SPECT studies of relatively small body organs, such as the heart and the brain, carried out using a SPECT-capable scintillation camera having more than one detector (a "multihead" scintillation camera).
In a nuclear medicine study of a body organ of interest, a radioisotope is administered to a patient and taken up in the body Organ. The patient is then placed in a scintillation camera, which has one or more detectors ("heads"). Each head contains a scintillator (usually a NaI(Tl) crystal). As the radioisotope undergoes radioactive decay, it gives off gamma radiation. This gamma radiation is collimated and directed in its collimated state to the scintillators. As the gamma radiation interacts with the scintillators, flashes of scintillation light ("scintillation events") occur in the scintillators. Photodetectors (conventionally, photomultiplier tubes or "PMTs") are optically coupled to the scintillators and respond to the scintillation events by producing electrical signals. Such signals, after appropriate signal processing, contain information which is used to form images that show where the radioisotope has been taken up. Such images are used by radiologists to identify pathology of the body organ which is the subject of the study.
For some diagnostic applications, the images are two-dimensional ones. Studies which produce such images are called planar imaging studies. In planar imaging studies, data is collected while the camera heads are fixed in position. However, most modern diagnostic applications require tomographic (three-dimensional) images of the distribution of the radioisotope (the "activity") within the organ of interest. Studies which produce such three-dimensional images are known as SPECT (Single Photon Emission Computed Tomography) studies. In conventional SPECT studies the camera heads are rotated around the patient by a gantry so as to sample the entire activity within and surrounding the organ of interest. With the information thus obtained, it is possible to use mathematical techniques (so-called "backprojection" techniques) to computer-reconstruct a three-dimensional SPECT image of the activity within the organ of interest.
The heart and the brain are relatively small as compared with the sensitive crystal surface in conventional scintillation camera detectors. It is therefore conventional practice to use focussing collimators (e.g. cone beam collimators, astigmatic collimators) when imaging such organs. This magnifies the image of the organ as "seen" by the detector and therefore increases sensitivity.
Where a SPECT study of a small body organ is carried out using only a focussing collimator (e.g. a cone beam collimator), image distortions ("reconstruction artifacts" ) appear in the reconstructed SPECT image. This is because a focussing collimator does not adequately sample the entire activity distribution around the organ of interest (even though a focussing collimator may adequately sample the entire activity distribution within the organ of interest.) Dr. Jaszczak et al. of Duke University have therefore proposed (see Three-Dimensional SPECT Reconstruction of Combined Cone-Beam and Parallel-Beam Data, Phys. Med. Biol. 37:535-548, 1992) that such studies be carried out using a cone beam collimator and a parallel hole collimator at the same time. Because the parallel hole collimator samples the entire activity distribution, the focussed and nonfocussed data can be combined to produce a SPECT image which not only benefits from the magnification afforded by the focussing collimator, but which nonetheless lacks the reconstruction artifacts which are caused by inadequate sampling.
Although this technique holds out the prospect of improving SPECT images produced by multi-head SPECT-capable scintillation cameras, SPECT studies are still lengthy. This is because constraints imposed by use of conventional cone beam collimation in SPECT studies limit sensitivity of the system. Substantial time is needed to acquire the necessary SPECT data.
It would therefore be advantageous to provide method and apparatus for increasing the sensitivity of SPECT studies carried out on multi-head scintillation cameras using both focussing and parallel hole collimators.
The invention proceeds from the realizations that a) a focussing collimator produces maximum sensitivity improvements when the organ of interest is as far away from the collimator as possible and b) a focussing collimator will adequately sample the activity within the organ of interest if the organ is always completely encompassed within the field of view of the collimator. It follows from these known facts that if this relationship could be maintained throughout the entire SPECT study, maximum use would be made of the sensitive crystal surface but truncation artifacts would not come about.
In accordance with the invention, a multi-head SPECT-capable scintillation camera has one detector which is collimated using a parallel hole collimator. Another detector, or more than one other detectors, are collimated using focussing collimators.
When a SPECT study of a small body organ such as the heart or the brain is to be conducted, the scintillation camera is used to acquire information about the organ. Based on this information, the organ of interest is located in space (with respect to the scintillation camera). Once the organ of interest has been located in space, the camera is operated in such a manner as to maintain the organ of interest in a predetermined optimum position with respect to the field(s) of view of the focussing collimator(s), throughout the entire rotation of the camera heads around the patient. In the normal case, this optimum position is the furthest possible distance from the camera head without causing the organ to project beyond the field(s) of view of the focussing collimator(s) and to thereby cause truncation artifacts.
Advantageously, and in accordance with a presently preferred embodiment of the invention, the location of the organ of interest is initially determined and the patient bed is subsequently moved so as to situate the organ of interest at a predetermined location within the scintillation camera. Once this has been done, a SPECT study is carried out in the normal manner, with appropriate radial movement of the camera heads to maintain the proper relative positioning of the organ with respect to the focussing collimators.
In the conventional case, the focussing collimator(s) are of the cone beam type because reconstruction algorithms have already been developed for this type of collimator when it is used together with a parallel hole collimator. However, where the detectors are rectangular, as in the MULTISPECT scintillation cameras manufactured by Siemens Gammasonics, Inc., owner of this and the above-stated two parent patent applications, it is especially advantageous if the focussing collimators are of the astigmatic type (see, e.g., commonly owned U.S. Pat. No. 4,670,657). This is because such collimators have two focal lines and these can be selected so as to optimally mate the characteristics of the collimator to the rectangular shape of the detectors.
To locate the organ of interest in space, the techniques disclosed in either or both of the above-referenced parent patent applications can be used.